Protocols for sharing a wireless medium effectively among multiple users are generally denoted multiple access protocols, channel access schemes or medium access schemes. For efficient (and often fair) sharing of the wireless medium, various channel access schemes have been developed over the years particularly targeted for distributed networks, such as multi-hop/ad-hoc networks.
Classical multiple access protocols may be divided in two main categories, conflict-free protocols and contention-based protocols.
Conflict-Free Multiple Access Protocols
Conflict-free protocols, which sometimes are referred to as scheduled channel-access protocols, ensure that a transmission, whenever made, is successful; i.e. not interfered by other transmissions. Conflict-free transmission can be achieved by allocating the channel to the users either statically or dynamically. This is often denoted fixed or dynamic scheduling respectively. The benefit of precise coordination among stations is that it is conceived to achieve high efficiency, but comes at the expense of complexity and exchange of sometime large quantities of control traffic.
In [1], Kleinrock and Sylvester suggested to schedule and reuse TDMA time slots spatially. The idea is to assemble groups (also called collision-free vectors or cliques) of links that can be used at the same time without causing mutually harmful interference. A number of such groups may be identified, and these groups are then cycled through in a TDMA-frame like manner. This scheme is generally denoted STDMA and stands for Spatial TDMA.
Contention-Based Multiple Access Protocols
Contention-based protocols differ in principle from conflict-free protocols in that a transmission is not guaranteed to be successful. The protocol should therefore prescribe a procedure to resolve conflicts once they occur so that all messages are eventually transmitted successfully.
A classical problem in packet radio networks, or ad hoc networks, is the existence of so-called hidden terminals. With reference to FIG. 1, the hidden terminal problem means that a node A transmitting to B is unaware of another node C's transmission to D (or possibly to B) which interfere with A's transmission at B. The result is obviously a collision at B that degrades performance in all aspects (throughput, delay and so forth). Means for tackling this problem has been suggested since the mid 1970's and consequently we will look at the classical “solutions” shortly. However, first note that CSMA [1] does not handle this problem since C and A per definition do not overhear each other's transmissions. Carrier sensing is therefore deemed unsuitable for packet radio networks [3]. In the worst case, the performance of CSMA degrades to the performance of ALOHA [1].
Various contention-oriented methods to handle the hidden terminal problem is described in the following:
Multiple Access With Collision Avoidance (MACA)
A method called MACA (Multiple Access With Collision Avoidance) by Karn [4] is based on sending a Request to Send (RTS) and Clear to Send (CTS) to ensure that neighbors of B are aware of whom will send. Assume that A issues an RTS, B responds with a CTS provided that it has received the RTS. Node A receives the CTS message and initiates the data transmission. C on the other hand refrains from transmitting anything as it has heard the CTS from Node B. Similarly, nodes in the vicinity of node, A which hears the RTS message refrain to transmit anything as A is waiting for the CTS message. A back-off scheme is employed for mitigating the impact of reoccurring collisions of RTS messages.
MACAW
In [5], Bhargawan et al. improved the MACA protocol and renamed it MACAW. They introduced link layer Acks as well as CSMA for the RTS messages. They also improved fairness by running the back-off scheme based on source-destination pair rather than node. Means for congestion control was also added. IEEE 802.11 now employs a very similar RTS-CTS scheme called DFWMAC in one of it's operating modes.
Busy Tone Multiple Access (BTMA)
A rather similar approach to MACA is the Busy Tone Multiple Access scheme, BTMA [6]. Rather than sending a CTS message, node B indicates that it is busy with a tone on some parallel channel (read other frequency). This can be done provided B has received its address content. However another and much less useful alternative proposed method is that all nodes detecting a packet transmission sends out a busy tone. The latter alternative will cause severe blockage of a large area. The practical use of either scheme has been rather limited and mostly flourished in academic papers.
Other Classical Multiple Access Protocols
Another medium access technique is based on Direct Sequence Code Division Multiple Access, DS-CDMA. In principle, two approaches are possible.
For example, referring once again to FIG. 1, mechanisms can be implemented which aim to ensure that node A and C uses orthogonal codes and hence does not interfere with each other.
Another approach is to exploit receiver directed spreading codes. The latter assumes that C sends its data towards a node D. It is noted that by using orthogonal codes, the available bandwidth resources are divided by the sender.
Multi-User Oriented Multiple Access Protocol
In [7], a multiple access protocol is described that combines STDMA and multi-user detection. In this method, transmissions are scheduled in time, space as well as in receive power. The transmit power levels are chosen such that multiple transmissions can be received concurrently and decoded through usage of a multi-user detector. The benefit is that the network throughput is improved over classical channel schemes.